1. Field of the Invention
The present invention relates to a developing apparatus which uses a two-component developer to develop an electrostatic latent image formed on an image carrier in an image forming apparatus, such as a copying machine, a printer, a recorded image display device, and a facsimile machine, using an electrophotographic method, an electrostatic recording method or the like.
2. Description of the Related Art
For a developing apparatus included in an image forming apparatus, a one-component developer having a non-magnetic toner or a magnetic toner as a main component, or a two-component developer having a non-magnetic toner and a magnetic carrier as main components, is used. Especially for a color image forming apparatus which forms a full color or a multi-color image by electrophotography, from the perspective of the tint and the like of the image, most developing apparatuses use a two-component developer.
Consequently, to stabilize an output image, it is very important to maintain the mixing ratio (hereinafter, “toner density”) of the non-magnetic toner and the carrier in the developing apparatus. Therefore, various methods have in the past been proposed as a method for replenishing the developer in the developing apparatus.
The following methods for controlling a toner replenishment amount are known. In one such example, a detection unit is provided near a photosensitive drum, a toner image (patch image) developed on the photosensitive drum is irradiated with light, and the transmitted light or reflected light there is detected by the detection unit. Then, based on the detection result, the toner replenishment amount is controlled.
In another example, the detection unit is provided near a developer carrier, and the toner density is detected based on the reflected light when the developer carried on the developer carrier is irradiated with light. Further, in another example, the toner density is detected by detecting changes in the apparent magnetic permeability of the developer in a fixed volume near a sensor utilizing the inductance of a coil. Still further, in another example, the amount of toner which will be consumed by an image is predicted based on a video count number of the density of an image obtained by reading an image information signal by a charge-coupled device (CCD) sensor or the like, and the toner amount corresponding to the predicted amount is replenished.
Among these examples, the method in which the density is controlled by a magnetic permeability sensor is used because the toner in the developing unit can be detected by a simple method without causing any downtime.
Generally, the output value of a magnetic permeability sensor which utilizes coil inductance decreases when the toner density increases, since the amount of carrier included in a fixed volume of developer decreases, causing the apparent magnetic permeability to decrease. Conversely, the output value of the sensor increases when the toner density decreases, since the amount of carrier included in a fixed volume increases, causing the apparent magnetic permeability to increase. To stably maintain the toner density in a developing apparatus having a magnetic permeability sensor, an appropriate amount of toner may be replenished by accurately detecting the toner density in the developing apparatus, and basing the replenishment amount on that detection result.
However, there are the following problems when controlling the toner density using a magnetic permeability sensor. As the arrangement location for the toner density sensor, it is desirable that the toner density sensor is in contact with the developer, the developer has a thickness and a surface which allows the toner density to be detected, and the flow of the developer near the toner density sensor face is uniform. However, the user has demanded in recent years that image forming apparatuses have a reduced size, and as a result, a size of the developing apparatus has been reduced. As a consequence, the arrangement location of the toner density sensor can be limited.
Normally, the developer is more easily retained due to the effects of gravity when the developer is closer to the bottom of the developing apparatus. Further, the developer which has entered a clearance that exists between the density sensor and the development container tends to be retained because there is no direct carrying unit. Therefore, compared with toner which is directly carried by a carrying member of a carrying screw, the toner at the clearance is not carried out as easily, and tends to be retained. A toner that is susceptible to retention like this may show a different density from the actual toner density because the toner cannot be easily replaced. Thus, there is the problem that the toner density cannot be detected with high accuracy.
Accordingly, Japanese Patent No. 3434118 discusses promoting stirring (developer replacement) of the developer by providing a rib on a shaft portion of a stirring/carrying member which opposes a magnetic permeability sensor face.
However, with a configuration such as that in Japanese Patent No. 3434118, there is the following problem. More specifically, the direction that the developer can be moved can be divided into two components, that is, a shaft direction of the developer stirring/carrying screw, and a circumferential direction of the developer stirring/carrying screw. When a rib is provided on the shaft of the stirring/carrying member as in Japanese Patent No. 3434118, the developer is carried in the circumferential direction of the stirring/carrying screw. In other words, the carrying direction of the developer in a stirring chamber 24 will be perpendicular to the circumferential direction. Therefore, the carrying rate in the screw rotational shaft direction of the developer at the portion facing the sensor face decreases.
In Japanese Patent No. 3434118, although the rib provided on the shaft of the stirring/carrying member promotes developer replacement, it also reduces the carrying rate of the developer (carrying speed of the developer) in the shaft direction at the portion facing the sensor face. The fact that the carrying rate of the developer decreases means that the developer which is present in the portion opposing the sensor face tends to stop at the same location, so that developer replacement in the portion opposing the sensor face becomes more difficult. Consequently, it becomes impossible to accurately follow the changes in the developer density in the developing apparatus. Further, if the developer is retained in the portion opposing the sensor, a surface of the developer opposing the sensor tends to be unstable. Therefore, if the surface of the developer fluctuates, height density at a detecting position of the sensor fluctuates, which may cause false detection. Thus, although providing a rib on the shaft of the developer stirring/carrying member has an effect in improving stirring properties, it has not been sufficient in terms of its effect on developer replacement.
On the other hand, while the carrying rate in the shaft direction could be increased for all areas, as a result, this would deteriorate the stirring properties of the developer in the developing apparatus. Thus, a problem would arise in terms of stirring (i.e., it becomes more difficult for the replenishment toner to mix with the developer, so that the toner density in the developing apparatus would not be uniform).